Fine structure of the external egg membrane of five species of Pacific salmon and steelhead trout

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Abstract
Fine structure of the external egg membrane of 5 species of Pacific salmon (sockeye, Oncorhynchus nerka; pink, O. gorbuscha; chum, O. keta; coho, O. kisutch; and chinook, O. tshawytscha) and the anadromous steelhead trout (S. gairdneri), is examined and compared using the scanning electron microscope. Membrane thickness in fixed material varies for the 6 spp. as follows (.mu.m, .hivin.x .+-. SE): sockeye, 34.15 .+-. 0.15; pink, 61.64 .+-. 1.53; chum, 53.05 .+-. 0.33; coho, 27.96 .+-. 0.48; chinook, 50.82 .+-. 0.74; steelhead, 30.74 .+-. 0.11. The membrane consists of a thin outermost layer, the externus, 0.2-0.3 .mu.m thick and the internus, 24-55 .mu.m thick, which constitutes the remainder of the membrane. In sockeye, pink and chum salmon and steelhead trout, an additional layer 3-8 .mu.m thick, the subinternus, occurs beneath the internus. The entire membrane appears fibrous except for the thin and apparently solid externus. Pores in both the inner and outer surfaces are arranged in a hexagonal pattern and are connected by pore canals traversing the membrane. Except in the sockeye, plugs commonly were seen blocking the external openings of the pore canals. Significance of the egg membrane fine structure is considered in relation to several of its roles in the water-activated egg: semipermeability, retention of internal pressure and mechanical protection. A structural and functional analogy is drawn between the fine structure of the salmonid egg membrane and the mammalian aorta.

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